PDE4D - 3D Mechanism Simulator

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References & Databases

START HERE — how to read cAMP + H₂O → 5′-AMP on PDE4D One hydrolysis cartoon on the human phosphodiesterase 4D catalytic domain: 1TB7 (1.63 Å) = PDE4D (UniProt Q08499), chain A, author residues 87–411. The deposited occupancy is the product 5′-AMP plus the dual-metal site Zn²⁺ (A1001) / Mg²⁺ (A1002) and the metal-bridging hydroxide O (HOH A1190). The substrate 3′,5′-cAMP is not modeled in 1TB7, so its pose is [schematic] — the cAMP ring geometry is transplanted from the human PDE4D–cAMP complex 2PW3 (1.56 Å) by a Cα overlay. On this crystal the bridging O–P–O3′ angle is 155° (near in-line) with O3′ as the leaving group, so the cut is (proposal, lit.-based). Zn²⁺ is held by His164 / His200 / Asp201 / Asp318; Mg²⁺ by Asp201 and waters; His160 clamps the phosphate. PDE4 is the hydrolase that switches cAMP off — the reverse chemistry of adenylyl cyclase (a separate page). Both metals stay. Author numbering is the canonical PDE4D2 numbering (+302 to UniProt). Measured Å are deposited 1TB7 pairs only.
Crystal structures PDB 1TB7 — catalytic domain of human phosphodiesterase 4D in complex with AMP, 1.63 Å; coordinates are public. Chain A = PDE4D (UniProt Q08499), author resi 87–411 (gap 292–297; the GSHM expression tag at 82–85 is excluded). Deposited metals: Zn²⁺ (A1001) and Mg²⁺ (A1002); bridging water HOH A1190; product AMP (A1003). EC 3.1.4.53 (3′,5′-cyclic-nucleotide phosphodiesterase). The substrate 3′,5′-cAMP ring is taken from 2PW3 — the human PDE4D–cAMP complex (D201N, 1.56 Å) — superposed onto this 1TB7 frame; that pose is still [schematic] (2PW3 is a catalytically-dead mutant).
Mechanism — metal-bridging hydroxide, in-line on the phosphate PDE4 hydrolyses the second messenger cAMP to the inactive 5′-AMP: 3′,5′-cAMP + H₂O → 5′-AMP. A hydroxide O bridging the Zn²⁺/Mg²⁺ pair is the nucleophile; it attacks the phosphate in-line with the leaving O3′ (O–P–O3′ = 155° on the crystal, near in-line). The metals polarise the phosphate and stabilise the pentacoordinate transition state. His160 clamps the phosphate; Zn²⁺ is ligated by His164 / His200 / Asp201 / Asp318. Source: PDB 1TB7 · UniProt Q08499 · BRENDA EC 3.1.4.53
0 — Setup: PDE4D active site (Zn/Mg + bridging OH⁻) 1TB7 holds the Zn²⁺ / Mg²⁺ pair (3.59 Å apart) bridged by the deposited hydroxide O (water A1190), the nucleophile. The pocket is shown empty so binding can start; 1TB7 models the product AMP, not cAMP. Opaque, no autoplay. The chemistry below is (proposal, lit.-based).Source: PDB 1TB7
① — 3′,5′-cAMP binds The second messenger 3′,5′-cAMP docks — [schematic], not a modeled state in 1TB7 (the crystal holds the product AMP). Its cyclic phosphate sits between the metals; His160 NE2 ⋯ P = 3.31 Å clamps it. The bridging hydroxide O is poised over the phosphate in-line with O3′. Measured register: Zn ⋯ His164 NE2 = 2.21 Å, Zn ⋯ His200 NE2 = 2.24 Å.Source: measured 1TB7 pairs · PDB 1TB7
② — bridging O rides onto the phosphate (in-line) (hypothetical, lit.-based) The metal-bridging hydroxide O (blue arrow) rides in from the Zn/Mg bridge and attacks the phosphate P in-line with the leaving O3′ (O–P–O3′ = 155° on the crystal), forming the new P–O bond. Zn²⁺/Mg²⁺ polarise the phosphate and stabilise the transition state. The blue arrow is the oxygen nucleophile’s path, not a hydride. Slow-motion on purpose.Source: compiled PDE chemistry · PDB 1TB7 geometry
③ — P–O3′ cleaves → 5′-AMP (hypothetical, lit.-based) Because the hydroxide O has now bonded to P, the P–O3′ bond breaks and the cAMP ring opens — the added O becomes a phosphate oxygen. The cyclic substrate gives way to the crystal 5′-AMP (phosphate carried forward, the attacking O now an AMP phosphate O; the free proton released as H⁺). The deposited pose is this product (P–O5′ = 1.61 Å; AMP O1P ⋯ Mg = 2.12 Å). Both metals stay. Slow-motion on purpose.Source: product pose from PDB 1TB7 · EC 3.1.4.53
④ — 5′-AMP leaves The product 5′-AMP — the crystal occupant of 1TB7 — departs, carrying the attacking O away; the Zn²⁺/Mg²⁺ pair remains. PDE4 has hydrolysed the second messenger to the inactive 5′-nucleotide. The product here is deposited (not schematic); the cAMP substrate was the schematic piece.Source: EC 3.1.4.53 net accounting
⑤ — Site restored — cAMP signal switched off The Zn²⁺/Mg²⁺ pair and the metal-bridging hydroxide are restored, ready for the next cAMP. Net: cAMP + H₂O → 5′-AMP (EC 3.1.4.53) — the phosphodiesterase step that terminates the cAMP second messenger. This is the reverse chemistry of adenylyl cyclase (a separate page). 1TB7 modeled the product AMP; cAMP was schematic.Source: EC 3.1.4.53 net accounting

How to read this page — a first-pass primer First pass: Setup (empty pocket + Zn/Mg + bridging O–H) → ① cAMP docks → ② the bridging O rides onto P in-line, forming the P–O bond → ③ P–O3′ breaks, cAMP → 5′-AMP → ④ AMP leaves with the attacking O → ⑤ metals + bridge restored. Second pass: 2.21 / 2.24 / 2.13 / 2.14 / 2.14 / 3.59 / 2.07 / 2.03 / 2.12 / 3.35 / 3.31 Å + 155° are crystal facts on 1TB7 atoms; cAMP, the attack and the proton bookkeeping are [schematic]. Third pass: open “Disagreements” for the product-vs-substrate occupancy, the transplanted cAMP pose, and the number of waters.
Chemistry primer — cyclic-nucleotide phosphodiesterase PDE4 is a hydrolase, not a cyclase: a metal-bridged hydroxide cuts the cyclic phosphodiester to the open 5′-nucleotide. Zn²⁺/Mg²⁺ = Lewis acids. The blue arrow = the hydroxide oxygen nucleophile’s path (not a hydride). Teaching substrate = 3′,5′-cAMP. This is not adenylyl cyclase (that makes cAMP, the opposite direction) and not a kinase/phosphatase.
Words used on screen — occupancy · cAMP · bridging hydroxide · in-line Occupancy — 1TB7 holds the product AMP, never relabeled as cAMP. cAMP — 3′,5′-cyclic AMP, the substrate here (schematic pose). Bridging hydroxide — the metal-bridging water O (deposited as HOH A1190) drawn as an explicit O–H, the nucleophile. In-line — the O–P–O3′ angle (155°) puts the attacking O opposite the leaving O3′ for an S_N2-style cut.

Legend & Fidelity

Mechanism fidelity — measured vs. inferred The measured layer: 1TB7 (1.63 Å) — Zn⋮Mg = 3.59 Å; Zn ⋯ His164 NE2 = 2.21 Å; Zn ⋯ His200 NE2 = 2.24 Å; Zn ⋯ Asp201 OD2 = 2.13 Å; Zn ⋯ Asp318 OD1 = 2.14 Å; Mg ⋯ Asp201 OD1 = 2.14 Å; bridging water O ⋯ Zn = 2.07 Å; bridging water O ⋯ Mg = 2.03 Å; AMP O1P ⋯ Mg = 2.12 Å; AMP P ⋯ Zn = 3.35 Å; His160 NE2 ⋯ AMP P = 3.31 Å; bridging water O–P–O3′ = 155.3° (near in-line); AMP P–O5′ = 1.61 Å. The inferred layer ([schematic]): cAMP substrate (2PW3 ring transplanted, pose schematic), the in-line attack, the proton bookkeeping and product release. Binding order is compiled staging, not a kinetic fit. The protein mesh is static. The product-vs-substrate occupancy is disclosed, not hidden.
Disagreements on record (a) Occupancy. 1TB7 models the product AMP, not cAMP; the substrate cAMP pose is [schematic]; its ring geometry is transplanted from the 2PW3 PDE4D–cAMP complex (D201N). (b) Nucleophile. The attacking hydroxide is the metal-bridging water O (HOH A1190); its protonation state is not resolved by X-rays. (c) The attack is a proposal. The 155° O–P–O3′ angle is near in-line but is a ground-state attack register, not a trapped transition state. (d) Number of metals/waters. PDE4 uses a dual Zn/Mg site; other PDEs use different metal combinations, and additional second-shell waters are not modeled here. (e) His160 role. Assigned as the phosphate clamp from the deposited contact; a distinct general-base residue is not assigned on this frame. (f) No protein-domain swing. (g) Gaps. Chain A has a gap (292–297) and a GSHM tag (82–85) excluded; only modeled Cα are drawn.
Regulation — PDE4 isozymes, cAMP compartmentation PDE4D is one of the cAMP-specific PDE4 family (PDE4A–D); the N-terminal regions (UCR1/UCR2) are regulatory and are not in this catalytic-domain construct. PDE4 inhibitors raise cAMP and are anti-inflammatory drugs, but the drug-bound soaks (1Y2K and the pyrazole series) are not this mesh — this page teaches the catalytic chemistry on the product complex 1TB7. Adenylyl cyclase makes cAMP; PDE4 destroys it — together they set the cAMP level.Source: UniProt Q08499 · PDB 1TB7
Kinetics scope (no measured timecourse on this page) One hydrolysis event is drawn, not a steady-state run. Compiled BRENDA values for EC 3.1.4.53 are not re-asserted here — no measured k_cat/K_m on this page [schematic] (qualitative only). Pair with a kinetics chapter.Source: BRENDA EC 3.1.4.53 — no k_cat/K_m re-asserted
Sequence Human PDE4D UniProt Q08499 · NCBI Gene 5144. Screen uses 1TB7 author numbering (chain A, 87–411); author numbering = canonical PDE4D2 numbering (+302 to UniProt; author 160 = UniProt 462). Zn ligands His164 / His200 / Asp201 / Asp318; Mg via Asp201; phosphate contact His160.
Simulation map — what you see is what is written Setup = empty 1TB7 pocket + Zn/Mg + bridging hydroxide (opaque, no autoplay). ① cAMP moves in (schematic). ② slow-mo: the bridging O rides onto P, forming the P–O bond. ③ slow-mo: P–O3′ breaks; cAMP fades to 5′-AMP, the added O now on the AMP phosphate. ④ AMP leaves with the attacking O. ⑤ metals + bridge restored. Only these events move.
Sources — bibliography of this page S1 — PDB 1TB7: catalytic domain of human PDE4D in complex with AMP, 1.63 Å — public coordinates; measured Zn⋮Mg = 3.59 Å · Zn⋮His164 NE2 = 2.21 Å · Zn⋮His200 NE2 = 2.24 Å · Zn⋮Asp201 OD2 = 2.13 Å · Zn⋮Asp318 OD1 = 2.14 Å · Mg⋮Asp201 OD1 = 2.14 Å · bridge O⋮Zn = 2.07 Å · bridge O⋮Mg = 2.03 Å · AMP O1P⋮Mg = 2.12 Å · AMP P⋮Zn = 3.35 Å · His160 NE2⋮P = 3.31 Å · bridge O–P–O3′ = 155.3° S2 — Sequence/annotation: UniProt Q08499 (human PDE4D) · NCBI Gene 5144 — Zn-binding His164 / His200 / Asp201 / Asp318 S3 — Kinetics: BRENDA EC 3.1.4.53 — no k_cat/K_m re-asserted on this page (qualitative) S4 — PDB 2PW3: human PDE4D–cAMP complex (D201N, 1.56 Å) — the real 3′,5′-cAMP ring transplanted onto the 1TB7 frame by Cα Kabsch (RMSD 0.27 Å); the pose stays [schematic]

› Backbones and ligand poses are taken directly from the listed PDB entries, superposed into one common frame. Where no crystalline state exists for a step (e.g., 3′,5′-cAMP substrate, the in-line attack, product release), the pose is drawn schematically and labeled as such in the narration.

PDE4D N-lobePDE4D C-lobeZn²⁺Mg²⁺ONPnucleophile (blue)